To produce white light using a blue LED die, it is well known to deposit a YAG phosphor, or red and green phosphors, directly over the led die by, for example, spraying or spin-coating the phosphor in a binder, electrophoresis, applying the phosphor in a reflective cup, or other means. It is also known to affix a preformed tile of phosphor (e.g., a sintered phosphor powder) on the top of the LED die. Such phosphor layers are non-remote since they directly contact the surface of the semiconductor die. Blue light leaking through the phosphor, combined with the phosphor light, produces white light. Problems with such non-remote phosphors include: 1) there is significant backscattering of blue light from the phosphor layer, which is then partially absorbed by the LED, submount, and metal electrodes; 2) there is a significant amount of light generated by the phosphor that is partially absorbed by the LED, submount, and metal electrodes; 3) the photon density is very high for high power LEDs and saturates the phosphor; 4) the LED is very hot and phosphors may react to the heat to cause darkening of the polymer binder layer (e.g., silicone) in which the phosphor particles are imbedded; and 5) due to the various angles of blue light rays passing through different thicknesses of phosphors (a normal blue light ray passing through the least thickness), the color varies with viewing angle.
It is also known to infuse phosphor powder in a silicone binder and mold the silicone over the LED die to form a lens, such as described in U.S. Pat. No. 7,344,902, by Grigoriy Basin et al., assigned to the present assignee and incorporated herein by reference. The phosphor is distributed at a very low density in the lens. Such a remote phosphor creates a relatively large light source, whose brightness per unit area is greatly reduced compared to a LED die with a thin coating of phosphor. Also, since the phosphor overlies a large area of the submount, the phosphor light is partially absorbed by the submount, as well as the LED die and electrodes, so the efficiency of the white light LED is reduced.
The paper entitled, “A Nearly Ideal Phosphor-Converted White Light-Emitting Diode,” by Allen et al., Applied Physics Letters 92, 143309 (2008), describes a bare LED die surrounded by an air gap and a hemispherical phosphor layer encapsulated by a transparent layer. There is poor light extraction from the LED into the air gap, and light is absorbed by the submount. It is also very difficult to economically manufacture the device.
What is needed is a technique to create a phosphor-converted LED, using a remote phosphor, that is very efficient by having less light absorbed by the LED and submount. It is also desirable to provide a remote phosphor where the resulting light source is smaller than a light source having phosphor infused in a silicone lens.